Optical receiving device

ABSTRACT

An optical receiving device has a photoelectric conversion layer including a matrix semiconductor containing silicon atoms as a main component, an n-type dopant D substituted for the silicon atom in a lattice site, and a heteroatom Z inserted into an interstitial site positioned closest to the n-type dopant D, in which the heteroatom Z has an electron configuration of a closed shell structure through charge compensation with the dopant D.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromprior Japanese Patent Application No. 2006-138023, filed May 17, 2006,the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to optical receiving devices based on afilled tetrahedral (FT) semiconductor.

2. Description of the Related Art

Silicon optical receiving devices are not only used in photo detectorsas unit elements but also widely applied to integrated devices such asCCD image sensors, COMS image sensors, and solar cells.

Unfortunately, it is very difficult for silicon optical receivingdevices to provide both high sensitivity (quantum efficiency) and highresponse speed compared to optical receiving devices comprisinggermanium with a narrow-gap or GaAs, which is a direct semiconductor(see S. M. Sze, Physics of Semiconductor Devices, Chapter 13, pp.754-760 (John Wiley & Sons, 2nd ed.).

A comparison of silicon with germanium and GaAs with respect to theirabsorption spectra indicates that silicon exhibits an absorptioncoefficient at least one order of magnitude lower than those ofgermanium and GaAs from a near-infrared region to a visible region inthe vicinity of the band edge. Further, for the silicon opticalreceiving device, it is known that there is a trade-off between twolight receiving characteristics of sensitivity and response speed owingto the low absorption coefficient of silicon. That is, increasing thethickness of a photocarrier generation layer increases the amount ofincident light absorbed to improve the sensitivity. However, at the sametime, the traveling distance of carriers increases to reduce theresponse speed. In contrast, reducing the thickness of the photocarriergeneration layer improves the response speed but lowers the sensitivity.

Thus, the small absorption coefficient of silicon is a direct cause ofthe trade-off between high sensitivity and fast response in the siliconoptical receiving device. A more essential cause is the fact thatsilicon is an indirect semiconductor with a wide band gap unlikegermanium or GaAs. Accordingly, it is difficult to solve the aboveproblem as long as silicon is used for the photocarrier generationlayer.

As described above, it is disadvantageously difficult for the siliconoptical receiving device to provide both high sensitivity and fastresponse. The direct cause is the low absorption coefficient of silicon,and the more essential cause is the fact that silicon is an indirectsemiconductor.

BRIEF SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided anoptical receiving device, comprising: a photoelectric conversion layercomprising a matrix semiconductor containing silicon atoms as a maincomponent, an n-type dopant D substituted for the silicon atom in alattice site, and a heteroatom Z inserted into an interstitial sitepositioned closest to the n-type dopant D, the heteroatom Z having anelectron configuration of a closed shell structure through chargecompensation with the dopant D.

According to another aspect of the present invention, there is providedan optical receiving device, comprising: a photoelectric conversionlayer comprising a matrix semiconductor containing silicon atoms as amain component, a p-type dopant A substituted for the silicon atom in alattice site, and a heteroatom Z inserted into an interstitial sitepositioned closest to the p-type dopant A, the heteroatom Z having anelectron configuration of a closed shell structure through chargecompensation with the dopant A.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a band diagram showing change in the band structure of siliconhaving isotropic stretching applied thereto;

FIGS. 2A and 2B show band diagrams of crystalline silicon and He-dopedFT-silicon, respectively;

FIG. 3 is a spectrum illustrating change in absorption coefficient ofsilicon into which an FT structure is introduced;

FIGS. 4A, 4B, and 4C are diagrams illustrating electron states in thereal space in respect of the Γ point conduction band, the X pointconduction band and the Γ point valence band of the energy bands ofsilicon;

FIG. 5 is an energy band diagram of silicon illustrating the reason whyan FT semiconductor more significantly absorbs light;

FIG. 6 is a diagram showing the structure of a pendant type FTsemiconductor;

FIGS. 7A and 7B show band diagrams of silicon having a PF pairconcentration of zero and a pendant type FT-Si having a PF pairconcentration of 6.3×10²¹/cm³, respectively;

FIGS. 8A and 8B are cross-sectional views showing the structures ofsilicon optical receiving devices of a vertical type and a lateral type,respectively, according to embodiments;

FIGS. 9A, 9B, 9C and 9D are cross-sectional views showing a method offorming a photocarrier generation layer (photoelectric conversion layer)of a PF-doped FT-Si according to an embodiment;

FIG. 10 is a graph showing response characteristics to input signals ofthe optical receiving device according to a first embodiment;

FIG. 11 is a graph showing response characteristics to input signals ofthe optical receiving device according to a second embodiment;

FIGS. 12A and 12B are a cross-sectional view and a circuit diagram,respectively, of the CMOS image sensor according to a third embodiment;

FIG. 13 is a cross-sectional view of the CCD image sensor according to afourth embodiment; and

FIGS. 14A and 14B are a plan view and a cross-sectional view,respectively, of the solar cell according to a fifth embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention use a filled tetrahedral (FT)semiconductor as a band engineering method of modulating the bandstructure of a semiconductor. The term “FT semiconductor” conventionallyrefers to a crystalline solid in which a rare gas atom or a diatomicmolecule with an electron configuration of a closed shell is introducedinto an “interstitial site” of a matrix semiconductor having atetrahedral structure such as a diamond structure or a zinc blendestructure. The effects of the FT semiconductor, the core of the presentinvention, will be described in detail.

Described in the first step are (1) the reason why an indirectsemiconductor such as silicon has an indirect band structure, and (2)the reason why the indirect semiconductor has a low absorptioncoefficient. Then described are (3) the feature of the FT semiconductor(rare gas-containing FT semiconductor and molecule-containing FTsemiconductor), and (4) the principle of enhanced absorption. Furtherdescribed is (5) a novel FT semiconductor, i.e., a pendant type FTsemiconductor, which constitutes the important part of the presentinvention.

(1) Band Structure of the Indirect Semiconductor:

FIG. 1 shows the band structure of silicon. Originally, the main reasonwhy silicon forms an indirect semiconductor resides in that the bondlength d between the adjacent Si atoms is relatively short. The energydifference ΔE between the conduction band and the valence band at the Γpoint is a function of the bond length d and can be representedapproximately by ΔE∝1/d². Therefore, the energy difference ΔE is rapidlydiminished with increase in the bond length d and is changed to beadapted to a direct band structure.

FIG. 1 shows, together with the band structure of a normal lattice, theresult of the calculation of the band structure of an imaginary lattice,covering the case where the lattice is stretched through a strain effectin the direction of the crystal axis <111> so as to increase the Si—Sibond length by 10%. In the drawing, the band structures of the normallattice and of the imaginary lattice are depicted to permit the upperedges of the valence bands to be matched.

As shown in FIG. 1, if the bond length is increased, the conduction bandis markedly dropped at the Γ point, though a marked change is notobserved in the X point, so as to be changed into a direct bandstructure resembling that of GaAs. Roughly speaking, the energydifference ΔE is diminished because the bond is elongated so as todecrease the repulsion energy between electrons, with the result thatthe conduction band (s-orbital) positioned upward in the normal latticeis lowered to approach the valence band (p-orbital). However, it islikely to be difficult to increase the bond length by the order of 10%.

(2) Optical Characteristics of an Indirect Semiconductor:

In the indirect semiconductor, the electric dipole transition isoptically forbidden. Thus, the indirect semiconductor exhibits only weakabsorption attributed to a phonon-assisted indirect transition in a lowenergy region near the band edge (i.e., it has a low absorptioncoefficient). In contrast to the above, a direct transition attributedto an electric dipole transition occurs in a direct semiconductor suchas GaAs, leading to significant absorption (i.e., it has a highabsorption coefficient). The difference between the two semiconductorsis mainly caused by whether the two selection rules given below aresatisfied.

One of the selection rules relates to the wave number, i.e., therequirement that the energy gap should be made smallest at the specifiedwave number. The other selection rule relates to symmetry of the wavefunction, i.e., the requirement that, in the wave number that makes thegap minimum, one of the conduction band and the valence band should bean even function and the other should be an odd function.

It should be noted in respect of the selection rule of the symmetry thatthe intensity of the emission and the absorption between two levels isgiven by <upper level|transition dipole moment μ|lower level>. For asemiconductor in which the two levels are represented by an s-orbital(even function) and a p-orbital (odd function) in the vicinity of anatomic orbital, μ corresponds to an odd function, so that the followingrelation is met, which means that this semiconductor is opticallyallowed.

<s|μ|p>=∫even·odd·odd dr≠0.

On the other hand, for a semiconductor in which the two levels arerepresented by the p-orbital, the following relation is met, which meansthat this semiconductor is optically forbidden:

<p|μ|p>=∫odd·odd·odd dr=0.

In the direct semiconductor, the gap is made minimum at the Γ point soas to satisfy the selection rule of the wave number. In the directsemiconductor, the wave functions of the conduction band and the valenceband are expressed by the s-orbital and the p-orbital, respectively,with the result that the selection rule of the symmetry is alsosatisfied.

On the other hand, in the indirect semiconductor, the conduction bandand the valence band differ from each other in the wave number makingthe gap minimum, resulting in failure to satisfy the selection rule ofthe wave number. In addition, since the wave functions for bothconduction band and valence band are represented by the p-orbital, theselection rule of the symmetry is not satisfied either. It follows thatthe indirect semiconductor is optically forbidden.

(3) FT Semiconductor:

The FT semiconductor is a theoretic material that was discovered in 1984in the process of calculating the conduction band structure of GaAs (seeH. W. A. M. Rompa et al., Phys. Rev. Lett., 52, 675 [1984] and D. M.Wood et al., Phys. Review B31, 2570 [1985]). Rompa et al., whodiscovered the theoretic substance, found through the band calculationthat FT-GaAs having He introduced into the interstitial sites of GaAsexhibits an increase in X point energy.

The present invention uses the FT semiconductor as a new bandengineering method and applies the FT semiconductor structure, whichallows the X point energy to be controlled, to an indirect semiconductorsuch as silicon. This imparts a significant light absorption to theindirect semiconductor, which originally exhibits only an insignificantlight absorption.

As described above, the term “FT semiconductor” conventionally refers toa crystalline solid in which a rare gas atom or a diatomic molecule withan electron configuration of a closed shell is introduced into an“interstitial site” of a matrix semiconductor having a tetrahedralstructure such as a diamond structure or a zinc blende structure.

Description will be given of the difference between the band structuresof the ordinary crystalline silicon and the FT semiconductor. FIG. 2A isa band diagram of crystalline silicon. FIG. 2B is a band diagram ofHe-doped FT-silicon. FIG. 2B shows the first principle band calculationin respect of silicon with an FT structure (hereinafter referred to asFT-silicon), in which a He atom is imaginarily inserted in theinterstitial site of crystalline silicon. As apparent from thesediagrams, the band structure of the FT-silicon is modulated into adirect transition type well resembling that of GaAs in which the shapeof the conduction band is widely varied from that of the crystallinesilicon. One of effects of the FT semiconductor is to significantlymodulate the indirect band structure of the indirect semiconductor,exemplified by silicon, into a direct one to sharply increase anabsorption coefficient for a part of absorption spectrum of silicon fromthe band edge toward a high energy side as shown in FIG. 3.

(4) Principle of Enhancement of Absorption in the FT Semiconductor:

FIGS. 4A, 4B, and 4C are show electron states in the real space inrespect of the Γ point conduction band (Γc), the X point conduction band(Xc), and the Γ point valence band (Γv), respectively, in the energyband of silicon.

As shown in FIG. 4A, silicon atoms are positioned at the atomiccoordinates (0, 0, 0) and (1/4, 1/4, 1/4) as viewed in the direction ofthe crystal axis <111> and bonded to each other by the Si—Si bond.Interstitial sites called tetrahedral sites are arranged at the atomiccoordinates (2/4, 2/4, 2/4) and (3/4, 3/4, 3/4). The tetrahedralstructure has a crystal structure having a relatively large clearance inwhich two atoms, two interstitial sites, and again two atoms arearranged along the crystal axis <111>. No atom is present in theinterstitial site. However, since the anti-bonding and bondingp-orbitals of the silicon atom are expanded toward the interstitialsite, an electron state is present in the interstitial site. In short,the state of the p-orbital is present in the interstitial site. Theprinciple of enhancement of absorption is based on the formation of anFT structure in the interstitial site with resultant selectivemodulation of the p-orbital.

In the well-known FT semiconductor, the FT structure is formed byintroducing a rare gas atom (or molecule) with an electron configurationof a closed shell into the space in the interstitial site. The FTstructure formed excludes an electron from the interstitial site toincrease the energy of Xc and Γv attributed to the p-orbital. However,the Γc energy attributed to the anti-bonding s-orbital is almostunaffected. This reduces, therefore, the difference between the Γcenergy and the Γv energy to lower the level of Γc relative to Γv,resulting in a direct transition. This increases the light absorption.

The above discussions will be summarized with reference to an energyband diagram shown in FIG. 5. As shown in this diagram, in thecrystalline silicon, the p-orbitals constitute the bottom of theconduction band and the top of the valence band. The s-orbital ispositioned upward in the conduction band. The formation of the FTstructure involves introducing the rare gas atom (or molecule) into theinterstitial site to raise the two p-orbitals closer to the s-orbitaland further to cause level crossing. An optically allowed s-p transitionexhibiting significant absorption is shifted toward a low energy side toimprove the absorption coefficient in a long wavelength region.

The presence of an atom in the interstitial site may form a deep ordefect level within the band gap, which may reduce a light current.However, in the FT structure, the atom (or molecule) of the closed shellstructure having a wide gap is inserted into the interstitial site,which prevents in principle the formation of such a level.

(5) Problems with the Rare Gas-Containing or Molecule-Containing FTSemiconductor:

However, the rare gas-containing or molecule-containing FT semiconductorproposed by Rompa et al. is believed to be thermally unstable becausethe inserted substance can move within the crystal and, thus, not to besuitable for practical use.

Concerning the FT semiconductor, the result of an experiment is reportedthat, if rare gas atoms are ion-implanted in a silicon wafer,photoluminescence (PL emission) is generated in the energy region in thevicinity of 1 eV, though the mechanism of the PL emission is notclarified (see N. Burger et al., Phys. Rev. Lett., 52, 1645 [1984]).However, if the wafer in which the rare gas atoms have beenion-implanted is annealed, the PL emission is caused to disappear,though the reason therefore is again not clear. It is believed that thedisappearance of PL emission is derived from the fact that, since therare gas atom is not chemically bonded with the silicon atom, the raregas atom is diffused within the silicon crystal and may be finallyreleased from the wafer.

Accordingly, even if the rare gas-containing or molecule-containing FTsemiconductor can be formed into the FT structure, the resultantstructure is easily expected to be poor in thermal stability. In short,there is a problem that the conventional FT semiconductor will not be apractical material system.

(6) Novel Pendant Type FT Semiconductor:

FIG. 6 shows a bonding state of atoms in a novel FT semiconductoraccording to an embodiment. The novel FT semiconductor is referred to asa pendant type FT semiconductor. The pendant type FT semiconductor,constituting the important part of the present invention, comprisessilicon atoms of a matrix semiconductor having a tetrahedral structure,an n-type dopant D (or p-type dopant A) substituted for a silicon atomin a lattice site, and a heteroatom Z inserted into an interstitial sitepositioned closest to the dopant D (or A). The heteroatom Z has anelectron configuration of a closed shell structure through chargecompensation with the dopant D (or A) and is ionized. Thus, an ionicbond is formed between the dopant D (or A) and the heteroatom Z,allowing the dopant D (or A) to pin the heteroatom Z. The pendant typeFT semiconductor of this particular structure permits improving thethermal stability, which is a problem with the rare gas-containing ormolecule-containing FT semiconductor. This is because, if the dopant D(or A) and the heteroatom Z are to be pulled away from each other,electrostatic interaction is exerted between them so as to generateforce for maintaining the ionic bond.

FIG. 6 shows a pendant type FT semiconductor in which phosphorus (P) asthe n-type dopant D and fluorine (F) as the heteroatom Z, inserted intothe interstitial site closest to the dopant D, are introduced intosilicon atoms forming a matrix semiconductor. The electron configurationof the P atom is 1s²2s²2p⁶3s²3p³, and that of the F atom is 1s²2s²2p⁵. Acharge compensation effect is exerted between these two atoms to form anionic P⁺—F⁻ bond (PF pair). The P⁺ ion is substituted for the siliconatom at the lattice point to change into a tetrahedral structure and isthus stabilized. The F⁻ ion becomes to have an electron configuration ofa closed shell structure like neon (Ne) and is thus also stabilized.

Where a pendant type FT semiconductor is to be realized by usingsilicon, it is possible to use an n- or p-type dopant, which has alreadybeen used in the actual LSI process, can be used as it is for the dopantD (or A). This facilitates the manufacture of the pendant type FTsemiconductor so as to reduce the manufacturing cost thereof.

For the pendant type FT semiconductor according to the embodiment,whether a light receiving function can be imparted to the indirectsemiconductor is important as in the case of the rare gas-containing ormolecule-containing FT semiconductor. FIGS. 7A and 7B show the resultsof band calculations based on the first principle in respect of aPF-doped FT-Si, in which phosphorus (P) is used as the dopant D andfluorine (F) is used as the heteroatom Z. The results cover two cases ofFIG. 7A where the number of PF pairs is zero (the PF concentration iszero, and the Si atom concentration is 5.0×10²²/cm³), and FIG. 7B wherethe number of PF pairs is one relative to seven Si atoms (the PFconcentration is 6.3×10²¹/cm³).

According to the results of calculations, in the case where the PF pairconcentration is zero shown in FIG. 7A, there is the lowest edge of theconduction band in the vicinity of Xc, which indicates an indirect bandstructure inherent in crystalline silicon. In the case where the PF pairconcentration is 6.3×10²¹/cm³ shown in FIG. 7C, the Xc is markedlyraised so as to cause the whole substance to be changed into a directband structure. These calculations indicate that the introduction of thePF pair changes the inter-band transition itself to be opticallyallowed, increasing absorption in the whole substance.

In conclusion, the pendant type FT semiconductor, like the raregas-containing or molecule-containing FT semiconductor, is considered toproduce the effect of band-modulating an indirect semiconductor into adirect semiconductor to sharply increase the absorption coefficient ofthe inter-band transition. The absorption coefficient is expected toincrease consistently with the pair concentration.

In the embodiments, combinations of the matrix semiconductor, dopant Dor A, and heteroatom Z contained in the pendant type FT semiconductorare exemplified as follows.

(1) The matrix semiconductor is selected from the group consisting ofIVb elemental semiconductors and IVb-IVb compound semiconductors, thedopant D is selected from the group consisting of Va elements and Vbelements, and the heteroatom Z is selected from the group consisting ofVIIb elements.

(2) The matrix semiconductor is selected from the group consisting ofIVb elemental semiconductors and IVb-IVb compound semiconductors, thedopant A is selected from the group consisting of IIIa elements and IIIbelements, and the heteroatom Z is selected from the group consisting ofIa elements and Ib elements.

Combinations of the matrix semiconductor other than the IVb elementalsemiconductor, the dopant D or A, and the heteroatom Z are exemplifiedas follows.

(3) The matrix semiconductor is selected from the group consisting ofIIIb-Vb compound semiconductors, the dopant D is selected from the groupconsisting of IVa elements and IVb elements and substituted for the IIIbatom at a lattice site, and the heteroatom Z is selected from the groupconsisting of VIIb elements.

(4) The matrix semiconductor is selected from the group consisting ofIIIb-Vb compound semiconductors, the dopant A is selected from the groupconsisting of IIa elements and IIb elements and substituted for the IIIbatom at a lattice site, and the heteroatom Z is selected from the groupconsisting of Ia elements and Ib elements.

(5) The matrix semiconductor is selected from the group consisting ofIIIb-Vb compound semiconductors, the dopant D is selected from the groupconsisting of VIa elements and VIb elements and substituted for the Vbatom at a lattice site, and the heteroatom Z is selected from the groupconsisting of VIIb elements.

(6) The matrix semiconductor is selected from the group consisting ofIIIb-Vb compound semiconductors, the dopant A is selected from the groupconsisting of IVa elements and IVb elements and substituted for the Vbatom at a lattice site, and the heteroatom Z is selected from the groupconsisting of Ia elements and Ib elements.

The matrix semiconductor can be exemplified as follows. An example ofthe IVb elemental semiconductor includes silicon. The IVb-IVb compoundsemiconductor is selected from the group consisting of SiC, GeC,Si_(x)Ge_(1-x) (0<x<1) and Si_(x)Ge_(y)C_(1-x-y) (0<x<1, 0<y<1,0<x+y<1). The IIIb-Vb compound semiconductor is selected from the groupconsisting of BN, BP, AlP, AlAs, AlSb and GaP.

The dopant D or A, and the heteroatom Z can be exemplified as follows.The Ia element is selected from the group consisting of Li, Na, K, Rband Cs. The IIa element is selected from the group consisting of Be, Mg,Ca, Sr, and Ba. The IIIa element is selected from the group consistingof Sc, Y, La and Lu. The IVa element is selected from the groupconsisting of Ti, Zr and Hf. The Va element is selected from the groupconsisting of V, Nb and Ta. The VIa element is selected from the groupconsisting of Cr, Mo and W. The Ib element is selected from the groupconsisting of Cu, Ag, and Au. The IIb element is selected from the groupconsisting of Zn, Cd, and Hg. The IIIb element is selected from thegroup consisting of B, Al, Ga, In and Tl. The IVb element is selectedfrom the group consisting of C, Si, Ge, Sn and Pb. The Vb element isselected from the group consisting of N, P, As, Sb, and Bi. The VIbelement is selected from the group consisting of O, S, Se and Te. TheVIIb element is selected from the group consisting of F, Cl, Br and I.

The optical receiving device according to an embodiment has aphotocarrier generation layer (photoelectric conversion layer)comprising an FT semiconductor. The positions of electrodes relative tothe photoelectric layer are not particularly limited. FIGS. 8A and 8Bare cross-sectional views showing the structures of silicon opticalreceiving devices according to embodiments. FIG. 8A shows a verticaltype optical receiving device. FIG. 8B shows a lateral type opticalreceiving device.

In the optical receiving device of the vertical type shown in FIG. 8A,an photocarrier generation layer 2 comprising FT-Si is formed on an n⁺region 1, and a p⁺ region 3 is formed in the photocarrier generationlayer 2. In other words, the n⁺ region 1 and the p⁺ region 3 are incontact with the photocarrier generation layer 2 so as to interpose thephotocarrier generation layer 2 therebetween. An n-electrode 4 isconnected to the n⁺ region 1, and a p-electrode 6 is connected to the p⁺region 3. The photocarrier generation layer 2 and p electrode 6 areinsulated with an insulating layer 5.

In this optical receiving device, light carriers (electrons and holes)generated in the photocarrier generation layer are drifted in thevertical direction to obtain electrons from the n electrode 4 via the n⁺region 1 and to obtain holes from the p electrode 6 via the p⁺ region 3,and thus a light current is produced.

In the optical receiving device of the lateral type shown in FIG. 8B, aburied oxide film 12 is formed in a semi-insulating silicon substrate11, and a photocarrier generation layer 13 comprising FT-Si is formed onthe buried oxide film 12. An insulating film 14 isolates thephotocarrier generation layer 13. An n⁺ region 15 and a p⁺ region 16 areformed in a surface area of the photocarrier generation layer 13 so asto sandwich the photocarrier generation layer 13 therebetween in thesame plane. An n-electrode 17 is connected to the n⁺ region 15, and ap-electrode 18 is connected to the p⁺ region 16.

In this optical receiving device, light carriers (electrons and holes)generated in the photocarrier generation layer 13 are drifted in thelateral direction to obtain electrons from the n electrode 17 via the n⁺region 15 and to obtain holes from the p electrode 18 via the p⁺ region16, and thus a light current is produced.

In both vertical and lateral type optical receiving devices, the buriedoxide film is formed for preventing current leakage. However, the buriedoxide film need not necessarily be formed if the current leakage can beprevented by any means such as the element structure, substrateresistivity, and circuit.

Each of FIGS. 8A and 8B shows the basic structure of the opticalreceiving device, and various structures are possible for specificoptical receiving devices. For example, the optical receiving deviceaccording to the embodiments can be used as a unit element. A pluralityof optical receiving devices may be integrated together on the samesubstrate to produce a CCD image sensor or a CMOS image sensor. Aplurality of optical receiving devices may be integrated together on thesame substrate to form a solar cell panel. Optical receiving devices,light emitting devices, and waveguides connecting them may be integratedtogether on the same substrate to produce an optical device array. Thesemodifications will be described below in detail.

A method of forming a photocarrier generation layer having an FTstructure will now be described with reference to FIGS. 9A, 9B, 9C and9D. In the following description, a photocarrier generation layer ofPF-doped FT-Si is formed.

A Si wafer 21 is prepared as shown in FIG. 9A. A prescribed dopingregion 22 of the Si wafer 21 is then doped with phosphorus (P) as ann-type dopant D, as shown in FIG. 9B.

As shown in FIG. 9C, fluorine (F⁺) as a heteroatom Z is ion-implantedinto a prescribed doping region 22 of the Si wafer 21 already doped withP. In the ion implantation step, optimization is made of energy, dose,surface orientation of the substrate, tilt angle, substrate temperature,and so forth. The F⁺ ion is expected to receive an excess electronpossessed by the P atom and an electron fed from ground through thesubstrate to become an F⁻ ion.

In the step shown in FIG. 9D, annealing is carried out to recrystallizethe lattice disturbed by the ion implantation to form an photocarriergeneration layer 23 comprising FT-Si. In the annealing process, theannealing temperature, annealing time, atmosphere, and so forth areadjusted to controllably replace the silicon atom at the lattice pointwith the P atom and to insert the F atom into the interstitial site. TheP atom is positioned at the lattice point. However, the F atom takes theelectron from the P atom to make the P atom electrically inactive. Patom thus provides an increased resistivity. The P atom and the F atomare tonically bonded together and are not dissociated from each other inspite of a temperature rise during the annealing. The p and F atoms thusmaintain a paired state.

Further, the other steps are carried out to enable the production ofsuch an optical receiving device as shown in FIG. 8A or 8B.

As described above, a photocarrier generation layer having an FTstructure can be formed in a matrix semiconductor by the methodemploying the combination of ion implantation and annealing.Alternatively, a photocarrier generation layer having an FT structurecan be formed by a combination of thermal diffusion and annealing. Aphotocarrier generation layer having an FT structure can also be formedby any other method.

If the dopant D at the lattice point is bonded to the heteroatom Z inthe interstitial site as in the case of the PF pair, an inherentvibration mode differing from the lattice vibration of the matrixsemiconductor is generated. As a result, it is possible to analyzedirectly the FT structure by infrared spectroscopy or Ramanspectroscopy. When it comes to an example of the PF pair, thecalculation of the standard vibration indicates that a vibration modeappears in the vicinity of the wave number of 150 to 200 cm⁻¹. In thisfashion, evaluation of the vibration mode provides one of effectivemeans of examining the presence of the FT structure.

As an indirect and simple method of detecting the presence of a DZ (orAZ) pair, it is possible to employ an electrical measurement such asresistance measurement or Hall measurement. In the case of using ann-type (or p-type) dopant, the substrate before doping the heteroatom Zin the interstitial site exhibits n-type or p-type conductivity and,thus, has a low resistivity. If the dopant D (or A) is paired with theheteroatom Z, charge compensation reduces free carriers to increase theresistivity of the substrate. Thus, it is possible to detect whether theDZ (or AZ) pair has been formed by comparing the resistances or thecarrier concentrations before and after the doping of the heteroatom Z.

The present invention will be described in more detail with reference tospecific embodiments.

First Embodiment

A silicon optical receiving device of the lateral type, which isconstructed as shown in FIG. 8B, will be described. A PF doped FT-Siphotocarrier generation layer is formed by using silicon as the matrixsemiconductor, the P atom as an n-type dopant D substituted for alattice site, and the F atom as a heteroatom Z inserted into aninterstitial site. The PF pair concentration is set to 5×10²¹/cm³. Theconcentrations of the P atoms and the F atoms are determined bysecondary ion mass spectroscopy (SIMS).

To determine whether a PF pair of a pendant type FT structure is formedin the photocarrier generation layer 13, it is effective to examine thevibration mode inherent in the PF pair, which can be detected bymicrospectroscopy of the photocarrier generation layer. A method foreasily checking the PF pair formation is to form a PF-doped regionhaving the same composition as that of the photocarrier generation layerand a region doped only with P on the surface of a high-resistivitysubstrate and then to compare these two doped regions for sheetresistance or carrier concentration. The formation of a PF pair leads tocharge compensation to increase the resistivity of the PF-doped regionabove that of the region doped only with P, while reducing the carrierconcentration of the PF-doped region below that of the region doped onlywith P.

As seen from the result of the band calculation in FIG. 7, the band gapof the PF-doped FT-Si is substantially equal to that of the crystallinesilicon. When the optical receiving device is irradiated with light withenergy equal to or greater than the band gap to optically excite thePF-doped FT-Si in the photocarrier generation layer, a light current isgenerated.

To effectively derive the light current which has been generated in thephotocarrier generation layer via the electrodes, a driving voltage V isapplied to between the n electrode 17 and the p electrode 18 (not shownin FIG. 8B). Supposing the open circuit voltage between the electrodesof the optical receiving device is V_(OC), a driving voltage should meetthe following formula: V<V_(OC). In contrast, in a case where V>V_(OC),external carriers from the electrode are injected into the photocarriergeneration layer to cancel and decrease the light current. Thus, thesetting of the operating voltage V is an important factor determiningthe device characteristics. The open circuit voltage V_(OC) can bedetermined by scanning the driving voltage so as to zero the lightcurrent (V=V_(OC)).

FIG. 10 shows the response characteristics of output light currentsobserved when optical signals with a wavelength of 850 nm modulated at10 GHz are input to the optical receiving device according to theembodiment. As is apparent from FIG. 10, output light currents havingthe same waveform are obtained in response to the input optical signals.Thus, the optical receiving device according to the embodiment enablesthe high-speed detection of near infrared light with the wavelength of850 nm, for which the crystalline silicon exhibits low spectralsensitivity.

As described above, the pendant type FT semiconductor modulating theenergy band is very effective for imparting a high light absorbingfunction to the photocarrier generation layer of the silicon-basedoptical receiving device to increase the operating speed and sensitivityof the optical receiving device.

COMPARATIVE EXAMPLE

A device is produced which has exactly the same configuration as that ofthe device according to the first embodiment except that the B atom isused as the heteroatom Z in place of the F atom. The B concentration isset to 5×10²¹/cm³, which is equal to the F concentration in the firstembodiment.

Optical signals with a wavelength of 850 nm modulated at 10 MHz areinput to this optical receiving device to examine the output currents.The optical receiving device in the comparative example provides onlylow output currents and fails to sense the optical signals.

The insufficient output currents are due to the position of the B atomin the crystal. As widely known in the art, the B atom is a typicalp-type dopant and is substituted for the lattice site, not theinterstitial site. Thus, the charge compensation between the B atom andthe P atom increases the resistivity of the photocarrier generationlayer. However, the pendant type FT structure is not formed.

Therefore, in order to induce a high light absorbing function by forminga pendant type FT structure to modulate the band structure, sufficientconsideration must be given in selecting the combination of the dopantsubstituted for the lattice site and the heteroatom inserted into theinterstitial site.

Second Embodiment

An optical receiving device having exactly the same configuration asthat in the first embodiment is produced except that the B atom, ap-type dopant, is used as the dopant D and the K atom is used as theheteroatom Z. The B concentration and K concentration as determined bySIMS are both 4×10²¹/cm³, and the BK pair concentration is estimated at4×10²¹/cm³.

To determine whether a BK pair of the pendant type FT structure isformed in the photocarrier generation layer, it is effective todetermine the vibration mode inherent in the BK pair. It can also bedetermined by using a simpler method based of the resistivity value orcarrier concentration.

FIG. 11 shows the response characteristics of output light currentsobserved when optical signals with a wavelength of 850 nm modulated at10 GHz are input to the optical receiving device according to theembodiment. As is apparent from FIG. 11, output light currents havingthe same waveform are obtained in response to the input optical signals.

As seen from the embodiment, even with the combination of the p-typedopant and the heteroatom Z, the operating speed and sensitivity of theoptical receiving device can be increased by forming a pendant type FTstructure in the photocarrier generation layer to enhance lightabsorption.

Third Embodiment

FIGS. 12A and 12B show a CMOS image sensor according to a thirdembodiment. FIG. 12A is a cross-sectional view, and FIG. 12B is acircuit diagram. The CMOS image sensor comprises pixel circuits (thecircuit is depicted in a region enclosed by a dashed line in FIG. 12B)integrated on the same p-type Si substrate 31. Each pixel circuitincludes a photocarrier generation layer 32 comprising an n-type region,an amplifying element 33 that amplifies optical outputs from thephotocarrier generation layer 32, a select transistor 34 that selectsthe pixels, and a reset transistor 35 that resets signal charges. Thephotocarrier generation layer 32 has basically the same structure asthat of the photocarrier generation layer shown in the first embodiment.The amplifying element 33, select transistor 34, and reset transistor 35are all MOS transistors. The select transistor 34 has a gate electrodeconnected to a perpendicular select line PSL and a drain connected to asignal line SIG. The reset transistor 35 has a gate electrode connectedto a reset line RL.

When optical signals containing red light and near infrared light with awavelength greater than 600 nm are selectively input to the CMOS imagesensor via a filter, well contrasted output images (electrical signals)are provided.

Thus, the CMOS image sensor according to the embodiment enablessensitive image pickup even with light having a wavelength greater than600 nm, for which the crystalline silicon exhibits only low spectralsensitivity.

Fourth Embodiment

FIG. 13 is a cross-sectional view of a pixel circuit in a CCD imagesensor according to a fourth embodiment. In the CCD image sensor, pixelcircuits are integrated on the same substrate. In FIG. 13, a p well 42is formed on an n-type Si substrate 41, and an n-type photocarriergeneration layer 43 is formed in the p well 42. The photocarriergeneration layer 43 has basically the same structure as that of thephotocarrier generation layer shown in the first embodiment. The n-typeregion 43 is connected to a read transistor 44. Light is input to then-type region 43 to generate signal charges, which are then read, viathe read transistor 44, onto a vertical CCD comprising a transferelectrode. A light shielding film 45 is formed on the read transistor 44with an insulating layer interposed.

When optical signals with a wavelength greater than 600 nm areselectively input to the CMOS image sensor through a filter, wellcontrasted output images (electrical signals) are provided.

Thus, the CCD image sensor according to the embodiment enables sensitiveimage pickup even with light having a wavelength greater than 600 nm,for which the crystalline silicon exhibits only low spectralsensitivity.

Fifth Embodiment

FIG. 14A and FIG. 14B show the cell structure of a solar cell accordingto this embodiment. FIG. 14A is a plan view, and FIG. 14B is across-sectional view. The solar cell comprises optical receiving devices(cells) integrated on the same substrate.

In the solar cell, an n⁺ layer 51, a photocarrier generation layer 52comprising FT-Si, and a p⁺ layer 53 are stacked. A back electrode 54 isformed on a back surface of the n⁺ layer 51. Lattice-shaped surfaceelectrodes 55 are formed on the surface of the p⁺ layer 53.Antireflection coatings 56 are formed in the areas surrounded by thesurface electrodes 55.

When the solar cell is irradiated with false sunlight to determineconversion efficiency, it is found to be 50%. This value is higher thanthe efficiency of a solar cell comprising crystalline silicon (20 to30%) or an amorphous silicon (10 to 15%).

Thus, the solar cell according to the embodiment can effectively absorbsunlight using the photocarrier generation layer with a high absorptioncoefficient, achieving a high conversion efficiency.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. An optical receiving device, comprising: a photoelectric conversionlayer comprising a matrix semiconductor containing silicon atoms as amain component, an n-type dopant D substituted for the silicon atom in alattice site, and a heteroatom Z inserted into an interstitial sitepositioned closest to the n-type dopant D, the heteroatom Z having anelectron configuration of a closed shell structure through chargecompensation with the dopant D.
 2. The device according to claim 1,wherein the n-type dopant D is selected from Vb elements and theheteroatom Z is selected from VIIb elements.
 3. The device according toclaim 2, wherein the Vb element is P atom and the VIIb element is Fatom.
 4. The device according to claim 1, further comprising a pair ofelectrodes between which the photoelectric conversion layer is disposed.5. The device according to claim 4, wherein supposing an open circuitvoltage between the electrodes is V_(OC), a driving voltage V meets thefollowing formula: V<V_(OC).
 6. A CCD image sensor comprising: thephotoelectric conversion layer according to claim 1; and a transferelectrode adjacent to the photoelectric conversion layer, thephotoelectric conversion layer and the transfer electrode being formedon a same substrate.
 7. A CMOS image sensor comprising: thephotoelectric conversion layer according to claim 1; and an amplifyingelement connected to the photoelectric conversion layer via a wire, thephotoelectric conversion layer and the amplifying element being formedon a same substrate.
 8. A solar cell comprising: a plurality of theoptical receiving devices according to claim 1 formed on a samesubstrate.
 9. An optical receiving device comprising: a photoelectricconversion layer comprising a matrix semiconductor containing siliconatoms as a main component, a p-type dopant A substituted for the siliconatom in a lattice site, and a heteroatom Z inserted into an interstitialsite positioned closest to the p-type dopant A, the heteroatom Z havingan electron configuration of a closed shell structure through chargecompensation with the dopant A.
 10. The device according to claim 9,wherein the p-type dopant A is selected from IIIb elements and theheteroatom Z is selected from Ia elements.
 11. The device according toclaim 10, wherein the IIIb element is B atom and the Ia element is Katom.
 12. The device according to claim 9, further comprising a pair ofelectrodes between which the photoelectric conversion layer is disposed.13. The device according to claim 12, wherein supposing an open circuitvoltage between the electrodes is V_(OC), a driving voltage V meets thefollowing formula: V<V_(OC).
 14. A CCD image sensor comprising: thephotoelectric conversion layer according to claim 9; and a transferelectrode adjacent to the photoelectric conversion layer, thephotoelectric conversion layer and the transfer electrode being formedon a same substrate.
 15. A CMOS image sensor comprising: thephotoelectric conversion layer according to claim 9; and an amplifyingelement connected to the photoelectric conversion layer via a wire, thephotoelectric conversion layer and the amplifying element being formedon a same substrate.
 16. A solar cell comprising: a plurality of theoptical receiving devices according to claim 9 formed on a samesubstrate.